1. Field of the Invention
The present invention relates to a heat-shrinkable envelope that is composed of at least one shrinkable component in the form of a planar layer, a reinforcing component applied thereon and a cover layer joined to the shrinkable component and the reinforcing component.
2. Description of the Prior Art
A heat-shrinkable article of the type generally set forth above is disclosed in the European application 0 299 439. A reinforcing component is thereby essentially composed of ordered, individual elements that extend in the form of a lattice. The application of the reinforcing elements occur such that the respective planar layers are already present in their cross-linked and stretched condition, so that the reinforcing components are not subjected to any stretching. The reinforcing elements are fashioned such that a shape change corresponding to the shrink of the shrinkable component is possible.
The German published application 37 07 301 discloses a heat-restorable article wherein the reinforcing elements are composed of short fibers (whiskers) of inorganic materials that are distributed in an irregular arrangement in the plastic. The manufacturing process for such an article sequences that a mixture of a thermoplastic synthetic and the fibers is produced, whereupon this mixture is extruded to form a band or tube. This band or tube is subsequently cross-linked, heated to a temperature above the crystal melting point of the synthetic, deformed while heated, and is subsequently cooled in the stretched or, respectively, distorted condition. The short fibers, therefore, are a constituent part of the extrusion mass from the very beginning and, therefore, of the later, shrinkable component. The disadvantage of such an arrangement resides in the fact that threads having a greater length can be introduced only with difficulty and that the extrusion tool must be designed from the very outset such that these additional component parts can be co-extruded. Moreover, there is the risk that the fibers will align in the extrusion direction during the extrusion process. There is also the possibility that the fibers will "bore out" of the surface and, in particular, will penetrate towards the exterior during shrinking. As a consequence of the extrusion process, an orientation of the reinforcing fibers result that can hardly be controlled or is extremely difficult to control. Moreover, the introduction of a larger proportion of fiber material makes the application of extremely-thin outside layers very difficult, whereby it must also be taken into consideration that the extruder worm must co-convey this fiber material. The lengths of the fibers are selected extremely short for this reason and should lie in the range of 0.5-5 mm, this being potentially too short for a particularly good and tea-resistant reinforcing arrangement.
The German published application 37 37 005 discloses a heat-shrinkable article in which threads extending transversely relative to the stretching and shrinking direction and lying parallel to one another are provided, these threads extending over the full length of the heat-shrinkable article (i.e., from one end to the other). Shorter fibers are strewn over this foundation of parallel, individual threads, these shorter fibers being taken from a reservoir and applied in an arbitrary distribution onto the surface of the carrier band provided with the threads. The fibers are composed of cross-linked polymers, i.e. fiber materials that are already cross linked are applied onto the carrier layer. Difficulties are thereby to be of concern under certain circumstances in that the short fiber pieces are applied onto an already-existing ply of continuous parallel threads and the reinforcement insert is lent a somewhat non-uniform structure overall as a result thereof. In the following rolling process, it is also difficult to join the cover layer and the base layer adequately well and reliably with this intrinsically, differently-structured reinforcing insert. With respect to the reinforcing insert, moreover, the manufacturing process occurs in two steps in that the parallel threads are first applied onto a carrier band of, for example, hot-melt adhesive and the delivery of the short fiber elements is subsequently undertaken via a separate scattering and shaking device.